Zirconium dioxide (ZrO.sub.2), either in the form of the naturally occurring mineral baddeleyite or in the form of synthetically produced products (with partially different material parameters than the raw material), is used in the ceramics industry, e.g. for producing zirconium dioxide ceramics and pigments based on zirconium silicate.
Zirconium dioxide with a content above 99% is obtained from ores containing baddeleyite by means of grinding, flotation and chemical treatment (acid wash). The quality of the zirconium dioxide is subject by nature to variations and the d.sub.50 value (50% value of the residual summation curve) is generally above 15 .mu.m, which limits the reactivity in the case of solid-state reactions and renders its utility problematic in the production of pigments. The d.sub.50 value can be lowered by grinding but at the same time the rubbings from the grinding unit such as e.g. Al.sub.2 O.sub.3 and MgO cause new problems.
The thermal decomposition of zirconium sand (ZrSiO.sub.4) with soda, with subsequent hydrolytic treatment of the sodium zirconium silicate produced and optional calcining and grinding, can result in zirconium dioxides with a d.sub.50 value (measured by means of laser diffraction) in a range between 2 and 5 .mu.m and whose BET surface is between 3 and 15 m.sup.2 /g. From a morphological standpoint, these products are essentially spherical particles and agglomerates.
A disadvantage of these products is their usually insufficient suitability for use in structural ceramics as well as, in particular, their high price, which sets very narrow limits to their use in the production of pigments.
Zirconium dioxide can also be obtained by the thermal splitting of zirconium silicate (ZrSiO.sub.4) with subsequent separation of the silica. The thermal splitting of zirconium silicate takes place at temperatures above approximately 1700.degree. C.-W. C. Butterman et al., Amer. Mineralogist 52 (1967), p. 884. ZrSiO.sub.4 begins to decompose at approximately 1760.degree. C. into tetragonal, solid ZrO.sub.2 and liquid SiO.sub.2. Above approximately 2400.degree. C., ZrO.sub.2 and SiO.sub.2 form a uniform liquid which solidifies by means of a rapid cooling off-the thermally split zirconium silicate obtainable in this manner is an amorphous silica phase in which zirconium dioxide crystals are embedded and can be separated from the SiO.sub.2 phase by flotation methods and/or leaching methods. Plasma methods and arc methods are also known for the thermal splitting of ZrSiO.sub.4.
In the plasma methods-see Great Britain Patent No. 1,248,595 and Ceramics, February 1974, p. 30-a curtain of pulverized zirconium sand is allowed to trickle through the flame of the plasma burner, during which the zirconium silicate is thermally split and then cooled off. A complete splitting of the ZrSiO.sub.4 requires the use of extremely finely ground zirconium powder. The grinding of the zirconium sand is energy-intensive and, in addition, foreign substances from the powder aggregate are typically entrained. The products obtainable in the plasma method contain ZrO.sub.2 crystals with a diameter of 0.1 to 0.2 .mu.m and a length of many .mu.ms (see Great Britain Patent No. 1,447,276) and exhibit a different morphology than that of the products in accordance with the invention.
According to another method, zirconium silicate is melted in an arc and allowed to solidify as a block, the melted body is then subsequently broken and ground-(see Gmelin's Handbuch der anorganischen Chemie (i.e., Gmelin's Handbook of Inorganic Chemistry), zirconium, volume 42 (1958), p. 56). The thermally split zirconium silicate produced in this manner contains zirconium dioxide with an average grain or particle size (d.sub.50 value) of approximately 15 to 20 .mu.m and a BET surface of approximately 0.5 m.sup.2 /g.
Alternatively, the melt can also be removed from an arc melting furnace and converted into a spherical product by means of cooling off in air-(see German Patent No. 26 35 030). An applicant of the present invention determined that the grain distribution of the zirconium dioxide in a thermally split zirconium silicate produced in this manner results in an average grain diameter-d.sub.50 value, determined by laser diffraction-of over 3 .mu.m and in a specific surface according to BET of approximately 2 m.sup.2 /g. The determination of the substance's characteristics took place here, as in the other instances, on zirconium dioxide obtained by leaching out the thermally split zirconium silicate with concentrated sodium hydroxide solution to a residual SiO.sub.2 content of below 0.5% by weight.
The utility of the zirconium dioxides derived from the thermally split zirconium silicates produced in accordance with the above described methods for the production of ceramic pigments is limited on account of the zirconium dioxides' relatively low specific surface and/or high d.sub.50 value and, in addition, a broad grain spectrum. Thus, much remains to be desired when such products are used regarding the color intensity and/or the color tone in a given pigment recipe.